1. Field of the Invention
The present invention relates to zoom lens systems, interchangeable lens apparatuses, and camera systems. In particular, the present invention relates to: compact and lightweight zoom lens systems having, as well as excellent optical performance, short overall length; and interchangeable lens apparatuses, and camera systems, each employing the zoom lens system.
2. Description of the Background Art
In recent years, interchangeable-lens type digital camera systems (also referred to simply as “camera systems”, hereinafter) have been spreading rapidly. Such interchangeable-lens type digital camera systems realize: taking of high-sensitive and high-quality images; high-speed focusing and high-speed image processing after image taking; and easy replacement of an interchangeable lens apparatus in accordance with a desired scene. Meanwhile, an interchangeable lens apparatus having a zoom lens system that forms an optical image with variable magnification is popular because it allows free change of focal length without the necessity of lens replacement.
Zoom lens systems having excellent optical performance from a wide-angle limit to a telephoto limit have been desired as zoom lens systems to be used in interchangeable lens apparatuses. Various kinds of zoom lens systems each having a negative lens unit located closest to an object side, and a multiple-unit construction have been proposed.
For example, Japanese Laid-Open Patent Publication No. 2008-191301 discloses an imaging optical system having a three-or-more unit construction of negative, positive and negative lens units or negative, positive and positive lens units. In the optical system, a lens unit having negative optical power is located on the object side relative to a diaphragm, and the lens unit having negative optical power has a cemented lens composed of a plurality of lenses which are cemented with each other. Partial dispersion ratio and Abbe number of at least one lens constituting the cemented lens are set forth.
Japanese Laid-Open Patent Publication No. 2008-233611 discloses a variable magnification optical system having a four unit construction of negative, positive, negative and positive lens units. In the optical system, at least the second lens unit and the third lens unit move when the magnification varies from a wide-angle limit to a telephoto limit, and there is an overlap between an area of movement of the second lens unit and an area of movement of the third lens unit. The first lens unit is composed of one negative lens and one positive lens, the second lens unit is compose of at least two lenses, and the third lens unit is composed of one negative lens. Shape factor of the negative lens in the first lens unit, a focal length of the third lens unit and an average refractive index of the positive lens in the second lens unit are set forth.
In each of the imaging optical system and the variable magnification optical system disclosed in the above-mentioned patent literatures, however, the construction of the lens unit located closest to the object side makes it difficult to reduce the overall length of lens system although optical performance is maintained at a certain level. Thus, size reduction which is recently demanded is not achieved.